Electron paramagnetic resonance, optical absorption and IR spectroscopic studies of the sulphate mineral apjohnite

Apjohnite, a naturally occurring Mn-bearing pseudo-alum from Terlano, Bolzano, Italy, has been characterized by EPR, optical, IR and Raman spectroscopy. The optical spectrum exhibits a number of electronic bands around 400 nm due to Mn(II) ion in apjohnite. From EPR studies, the parameters derived, g = 2.0 and A = 8.82 mT, confirm MnO(H2O)5 distorted octahedra. The presence of iron impurity in the mineral is reflected by a broad band centered around 8400 cm−1 in the NIR spectrum. A complex band profile appears strongly both in IR and Raman spectra with four component bands around 1100 cm−1 due to the reduction of symmetry for sulphate ion in the mineral. A strong pair of IR bands at 1681 and 1619 cm−1 with variable intensity is a proof for the presence of water in two states in the structure of apjohnite.